Projector having an illuminance detector

ABSTRACT

The invention provides a projector having a light source, a display device to modulate light emitted from the light source, a lens device to project light emitted from said light source via said display device, wherein said lens device has a diaphragm mechanism formed having an aperture, an ambient light illuminance detector to measure illuminance of ambient light, a projected image illuminance detector to measure illuminance of an image projected on the screen, an illuminance comparator to compare an illuminance detected by said ambient light illuminance detector and an illuminance detected by said projected image illuminance detector, and a diaphragm mechanism/image signal controler to control an intensity of image signals input into the display device and to control a size of an opening of the aperture of the diaphragm mechanism in accordance with a result of comparison by the illuminance comparator.

REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 10/703,166, filed Nov. 7,2003, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a projector for projecting light thathas been modulated by a display device.

BACKGROUND OF THE INVENTION

Various projectors that are adapted to guide via a lighting opticalsystem to a display device, such as liquid crystal panels, light emittedfrom a light source and project said light onto a screen via a lensdevice after modulating the light by the display device are knownconventionally (e.g. Japanese Unexamined Patent Application PublicationNo. Hei-8-43729). In order to improve the contrast of a projected image,the lens device of a conventional projector may be provided with adiaphragm mechanism which has a circular aperture and is positionedwithin a group of lenses.

In the case of projectors that use liquid crystal panels or the like asthe display device, it is necessary to ensure telecentricity between thedisplay device and the lens device. Therefore, in order to improve thecontrast of a projected image while limiting the reduction oftelecentric properties, the entire lens device has to be designed topermit adjusting of the aperture diameters of the lens frames thatsecure the respective lenses.

SUMMARY OF THE INVENTION

However, setting the aperture diameters of all the lens framessupporting the respective lenses of the entire lens device iscomplicated and makes the reduction of production costs difficult.

In order to solve the above problem, an object of the present inventionis to provide a lens device and a projector which facilitate theimprovement of image quality.

The lens device of this invention is a lens device of a projector forprojecting light that has been modulated and emitted by a displaydevice, wherein the lens device is arranged telecentric to rays of lightemitted from said display device and provided with a diaphragm mechanismhaving an aperture at such a position that the principal rays emittedfrom various locations on the display device intersect with the opticalaxis.

With such a configuration, this lens device is capable of increasing theF-number (in other words reducing the view angle) so as to shut out thelight that passes near the perimeter of the lens, which is the areaprone to the influence of lens aberration. By thus forming a projectionimage of the light that passes near the center of the lens and istherefore less prone to lens aberration, this lens device is capable ofimproving the contrast of the projected image. As the lens device isarranged so as to be telecentric to rays of light emitted from thedisplay device, the principal rays of light emitted from variouslocations of the display device converge at a single point and intersectwith the optical axis. Providing the diaphragm mechanism with anaperture at such a position that the principal rays intersect with theoptical axis permits rays of light emitted from various locations on thedisplay device to be shut out in a symmetrical manner with respect tothe optical axis, thereby limiting reduction in telecentricity and,consequently, easily ensuring both telecentricity and improved contrast.

In the lens device of this invention, the shape of the aperture of thediaphragm mechanism is determined in accordance with contrastcharacteristics with respect to directions and angles of incidence tothe display device so as to ensure a sufficiently high contrast.Compared with a configuration that includes a circular aperture, such alens device is capable of limiting reduction in illuminance of aprojected image and also easier to improve the contrast of a projectedimage. The diaphragm mechanism may also be designed such that the sizeof its aperture is adjustable. The configuration described abovefacilitates adjustment of the contrast and the brightness of a projectedimage.

A projector according to this invention includes a light source, adisplay device adapted to modulate light emitted from the light source,and a lens device of this invention as described above which is adaptedto project light emitted from said light source via the display device.As a result of inclusion of such a lens device, this projector easilyensures both telecentricity and improved contrast.

A projector of this invention includes a light source, a display deviceadapted to modulate light emitted from the light source, a lens devicewhich is provided with a diaphragm mechanism and adapted to projectlight emitted from said light source via said display device, an imagesignal intensity detecting means for detecting and evaluating theintensity of image signals to be input into the display means, adiaphragm mechanism drive control means adapted to drive the diaphragmmechanism so as to reduce the size of the aperture of the diaphragmmechanism when the image signal intensity detecting means ascertainsthat the intensity of the image signal is lower than a given level, andan image signal control means adapted to increase the intensity of theimage signal when the image signal intensity detecting means ascertainsthat the intensity of the image signal is lower than a given level.

Should the intensity of image signals input into the display device beat a low level, a projector having the configuration described above iscapable of increasing the contrast of a projected image whilemaintaining its illuminance by reducing the size of the opening of theaperture of the diaphragm mechanism and increasing the intensity of theimage signals.

A projector claimed of this invention includes a light source, a displaydevice adapted to modulate light emitted from the light source, a lensdevice which is provided with a diaphragm mechanism and adapted toproject onto a screen light emitted from said light source via saiddisplay device, an object detecting means for detecting an objectbetween the lens device and the screen, and a diaphragm mechanism drivecontrol means adapted to drive the diaphragm mechanism so as to reducethe size of the opening of the aperture of the diaphragm mechanism whenthe object detecting means detects an object between the lens device andthe screen. Should there be an object between the lens device and thescreen, a projector having this configuration is capable of limiting theimpact light has on the object by reducing the dimension of the apertureof the diaphragm mechanism.

A projector of this invention includes a light source, a display deviceadapted to modulate light emitted from the light source, a lens devicewhich is provided with a diaphragm mechanism and adapted to project ontoa screen light emitted from said light source via said display device,an ambient light illuminance detecting means for measuring ambient lightilluminance, a projected image illuminance detecting means for measuringthe illuminance of an image projected on the screen, an illuminancecomparing means for comparing an illuminance detected by the ambientlight illuminance detecting means and an illuminance detected by theprojected image illuminance detecting means, and a diaphragmmechanism/image signal control means for controlling intensity of imagesignals input into the display device and the size of the opening of theaperture of the diaphragm mechanism in accordance with a result ofcomparison by the illuminance comparing means. Should the illuminance ofan image projected on the screen not be sufficiently intense withrespect to the illuminance in the room, this projector is capable ofreducing the size of the opening of the diaphragm mechanism so as toincrease the contrast of the projected image and setting the intensityof the image signals at a high level so as to limit reduction in theilluminance of the projected image. The lens device may be arranged soas to be telecentric to rays of light emitted from the display deviceand provided with a diaphragm mechanism whose aperture is positioned sothat the principal rays of light emitted from the display deviceintersect with the optical axis.

As the lens device is arranged telecentric to rays of light emitted fromthe display device, the principal rays of light emitted from variouslocations of the display device converge at a single point and intersectwith the optical axis. Therefore, by providing the diaphragm mechanismwith an aperture at such a position that the principal rays intersectwith the optical axis, rays of light emitted from various locations onthe display device can be shut out in a symmetrical manner with respectto the optical axis so that reduction in telecentricity is limited.Thus, this projector easily ensures both telecentricity and improvedcontrast. In this type of projector, the shape of the aperture of thediaphragm mechanism is determined in accordance with contrastcharacteristics with respect to directions and angles of incidence tothe display device so as to ensure a sufficiently high contrast.Compared with a configuration that includes a circular aperture, thisprojector facilitates improvement of the contrast of a projected imagewhile limiting reduction in illuminance of the projected image. Thisprojector may also include a display device which is comprised of liquidcrystal panels. With the configuration as above, such a projectorfacilitates the improvement of image quality by improving the contrastof the image while ensuring telecentricity even in cases where theprojector includes liquid crystal panels having contrast characteristicsthat are polarized with respect to the direction and angle of incidence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a part of an embodiment of aprojector provided with a lens device according to the presentinvention.

FIG. 2 is a schematic illustration of a first embodiment of a diaphragmmechanism of said lens device.

FIG. 3 is a schematic illustration of a second embodiment of a projectorof the invention.

FIG. 4 is a schematic illustration of a fourth embodiment of a projectorof the invention.

FIG. 5 is a schematic illustration of a fifth embodiment of a projectorof the invention.

FIG. 6 shows a measurement system for measuring the field anglecharacteristics of liquid crystal panels of said projector.

FIG. 7 shows another measurement system for measuring the field anglecharacteristics of said liquid crystal panels.

FIG. 8 shows said measurement system for measuring the field anglecharacteristics of said liquid crystal panels.

FIG. 9 is a graph showing the relationship between optical angles andrelative light intensities of rays of light entering said liquid crystalpanels.

FIG. 10 is a graph showing results of measurement of the field anglecharacteristics of said liquid crystal panels.

FIG. 11 is a schematic illustration of diaphragm mechanism to explain asixth embodiment of a projector of the invention.

FIG. 12 is a schematic illustration of diaphragm mechanism to explain aseventh embodiment of a projector of the invention.

FIG. 13 is a schematic illustration of diaphragm mechanism to explain aneighth embodiment of a projector of the invention.

FIG. 14 is a schematic illustration of diaphragm mechanism to explain aninth embodiment of a projector of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Next, a lens device and a projector according to an embodiment of thepresent invention are explained hereunder, referring to relevantdrawings.

Referring to FIG. 1, numeral 1 denotes a projector, which includes alight source (not shown), three liquid crystal panels (LCD) 3, and anoptical system. The liquid crystal panels 3 serves as a display device,which functions as a modulating means to modulate light emitted from thelight source. The optical system functions to project light emitted fromthe light source via the liquid crystal panels 3 onto a screen S. Theoptical system has a lighting optical system and a lens device 6. Thelighting optical system includes a dichroic mirror or a similar deviceto guide to the liquid crystal panels 3 light emitted from the lightsource. The lens device 6 serves as a projection optical system forprojecting onto the screen S rays of light emitted from the liquidcrystal panels 3. The lens device 6 has a dichroic prism 8 and aprojection lens 10. The dichroic prism 8 serves to combine color light,i.e. red (R), green (G), and blue (B), emitted from the plural number ofliquid crystal panels 3. The projection lens 10 serves to enlarge andproject light that has exited from the dichroic prism 8.

The projection lens 10 includes a lens barrel 11, in which the firstthrough fifth lens L1-L5, a diaphragm mechanism 14, and the sixththrough eleventh lens L6-L11 are arranged along an optical axis B inthis order from the end that is close to the screen S.

The lenses L1-L11 are arranged telecentric to light emitted from theliquid crystal panels 3. As a result, of the rays of light that havepassed through various locations, i.e. pixels, on the liquid crystalpanels 3, principal rays A, which are rays of light orthogonal to thesurfaces of the liquid crystal panels 3, converge at a point G on theoptical axis B. In other words, the principal rays A, which are rays oflight emitted parallel to the optical axis B, intersect with the opticalaxis B at the point G.

As shown in FIGS. 1 and 2, the diaphragm mechanism 14 of the projectionlens 10 includes a shield section 15 for blocking light, an aperture 16for passing light, and an operating means 18. The aperture 16 ispositioned so that the optical axis B passes through the aperture 16 andthat the point G, at which the principal rays A converge, is located inthe aperture 16. The aperture 16 does not have a circular shape; itsshape is determined in accordance with field angle characteristics ofthe liquid crystal panels 3, in other words contrast characteristicswith respect to directions and angles of incidence. In the case of thepresent embodiment, the aperture 16 is formed in the shape of anequilateral triangle. The diaphragm mechanism 14 is designed so that thesize of the opening of its aperture 16 can be adjusted by operating theoperating means 18 manually or automatically so as to adjust the shieldsection 15 and the aperture 16. In the case of the embodiment, theshield section 15 and the aperture 16 are adjusted so as to change thesize of the aperture 16 without changing its shape.

As described above, the projection lens 10 of the embodiment is providedwith the diaphragm mechanism 14. Therefore, by increasing the F-numberof the projection lens 10 (in other words by reducing the view angle),more of the stray light inside the lens barrel 11, i.e. the illuminatinglight reflected by the inner surface of the lens barrel 11, is permittedto enter, and the light that passes near the perimeter of the lens,which is the area prone to the influence of lens aberration, is shutout. As a result, a projection image is formed of the light that passesnear the center of the lens and is therefore less prone to the influenceof incidence of stray light or lens aberration. The embodiment describedabove is thus capable of providing an image with an improved contrast.Limiting the amount of light by means of the diaphragm mechanism 14results in darker projection images.

The diaphragm mechanism 14 for adjusting contrast is disposed at such alocation that the principal rays A intersect with the optical axis B.This configuration enables the appropriate disposition of the diaphragmmechanism 14 without impairing its telecentric property. With aprojection type display, particularly in the case of a projector 1 whichuses liquid crystal panels 3 as the display device, it is important toensure the telecentricity of light between the display device and theprojection lens 10. Therefore, the projection lens 10 of the lens device6 has such a structure that the principal rays A of the light emittedfrom the pixels of the liquid crystal panels 3 are telecentric, in otherwords parallel to the optical axis B. As the diaphragm mechanism 14 ofthe embodiment described above is disposed at such a location that theprincipal rays A intersect with the optical axis B of the opticalsystem, rays of light emitted from the pixels can be shut out in arelatively symmetrical manner with respect to the optical axis B. Forexample, the upper light beam and the lower light beam from each pixelcan be shut out symmetrically with respect to the optical axis B.Because of this feature, the lens device 6 of the embodiment is capableof improving contrast of projected images without reducing thetelecentric property.

As a single diaphragm mechanism 14 is sufficient to obtain a largeF-number, it is easy to achieve two things at the same time, i.e.setting F-number (in other words contrast) and ensuring telecentricity,without the necessity of adjusting the aperture diameter of each lensframe of the entire lens device. The present embodiment is thereforeeffective in reducing production costs of the lens device.

As the diaphragm mechanism 14 is provided with an operating means 18that is capable of adjusting the size of the opening of the aperture 16,a desired image quality can be easily achieved by adjusting the balancebetween the contrast and the brightness of the image when the device isin use or when the aperture is adjusted in the production process or inany other occasion. The operating means 18 may be provided in variousforms; for example, it can be formed as a lever to be rotated around thelens barrel 11.

The shape of the aperture 16 of the diaphragm mechanism 14 is determinedin accordance with field angle characteristics of the liquid crystalpanels 3, i.e. contrast characteristics with respect to directions andangles of incidence (in the case of the present embodiment, the aperture16 is formed in the shape of an equilateral triangle). This featureoffers improved contrast while limiting reduction in illuminance of aprojected image.

The diaphragm mechanism 14 may be provided with a driving means, such asa motor, so that the size of the opening of the aperture 16 can beadjusted by using the driving force of the driving means. An explanationis given of the structures of the second through fifth embodiments ofthe invention, each of which has such a driving means to drive thediaphragm mechanism 14.

Although not shown in the drawings, the second embodiment calls for theoperator of the projector 1 to drive a motor (not shown), which servesas the aforementioned driving means, by operating a driving switch ofthe motor while watching an image projected on the screen S so as toachieve a desired brightness and contrast of the projected image toimprove the image quality.

The third embodiment of the invention calls for improving the imagequality by automatically adjusting the size of the aperture 16 of thediaphragm mechanism 14 and the intensity of image signals in accordancewith the intensity level of image signals and brightness of an imageprojected on the screen S.

As shown in FIG. 3, the projector 1 of the third embodiment includes animage signal control circuit 21, a diaphragm mechanism drive controlcircuit 22, an image signal intensity detecting circuit 23, and aprojected image illuminance detecting circuit 25. The image signalcontrol circuit 21 serves as an image signal control means forcontrolling image signals to be input into the liquid crystal panels 3,which collectively serve as a display device. The diaphragm mechanismdrive control circuit 22 serves as a diaphragm mechanism drive controlmeans for changing the area of the aperture 16 of the diaphragmmechanism 14 by controlling the diaphragm mechanism 14. The image signalintensity detecting circuit 23 serves as an image signal intensitydetecting means for detecting and evaluating the intensity of imagesignals to be input into the liquid crystal panels 3. The projectedimage illuminance detecting circuit 25 serves as a projected imageilluminance detecting means for detecting brightness of images projectedon the screen S via photoelectric elements 24. The photoelectricelements 24 serve as a light receiving section. According to theintensity level of image signals detected by the projected imageilluminance detecting circuit 25, the image signal control circuit 21controls the intensity of image signals input into the liquid crystalpanels 3, and the diaphragm mechanism drive control circuit 22 adjuststhe size of the opening of the aperture 16 by driving the motor of thediaphragm mechanism 14.

The projector 1 of the third embodiment described above is designed suchthat when the intensity of the image signals on the screen S except forthe perfect black portion, in other words the image signals that form anprojected image, is lower than a given level, the motor is driven so asto reduce the size of the opening of the aperture 16 of the diaphragmmechanism 14, while the intensity of the image signals input to theliquid crystal panels 3 is increased so that the illuminance of theprojected image, which has decreased as a result of the reduction of thesize of the opening of the aperture 16, is returned to the level ofilluminance prior to the reduction of the opening.

The term “the perfect black portion” mentioned above refers to the partof the screen S that corresponds to the part displayed as “black” on theliquid crystal panels 3, which is the portion where light has passedthrough liquid crystal cells of the liquid crystal panels 3 but failedto pass through analyzers. In cases where the part displayed as “black”on the liquid crystal panels 3 is capable of completely shutting outlight from the light source, the illuminance of the perfect blackportion depends on the brightness of the ambient light around the screenS, such as light in the room. In reality, however, the liquid crystalpanels 3 are unable to shut out light completely even in a black-displaystate, and leaked light is projected onto the screen S. As a result, theperfect black portion is in such a state as to be illuminated vaguely byprojected light.

Should the image signal control circuit 21 determine that approximately95% or more of the image signals, in other words more than 95% of theliquid crystal cells that are being driven by the image signals, arebeing driven at an intensity level lower than 70% of the maximumintensity level, the image signal control circuit 21 stores theilluminance of the projected image at the moment, i.e. the illuminanceprior to reduction of the size of the opening of the aperture 16, andthe diaphragm mechanism drive control circuit 22 drives the motor sothat the size of the opening of the aperture 16 becomes 70% of the areawhen the aperture 16 is fully open. After the size of the opening of theaperture 16 reaches 70% of the area of the fully open aperture 16, theimage signal control circuit 21 increases the intensity of the imagesignals so that the illuminance of the projected image detected by theprojected image illuminance detecting circuit 25 becomes the same as theilluminance prior to the reduction of the size of the opening of theaperture 16.

Rather than depending on the illuminance of a projected image, controlmay be performed by increasing the overall intensity of the imagesignals by 10% of the average intensity of the image signals.

Another way of control is to limit the part that exceeds the maximumlevel within 5% of the entire image signals when increasing the level ofthe image signals. When the overall intensity of the image signals isincreased, the intensity of a part of the image signals that is alreadyat a high level reaches the maximum level. As a result, the imageprojected by the light that has passed through the liquid crystal cellscorresponding to such a part has no hue, in other words, it is white.Such a state is called halation and impairs the quality of the projectedimage. Therefore, in order to reduce halation and thereby preventreduction in the image quality, the image signals are controlled so asto limit the part where the intensity of the image signals exceeds themaximum level, for example within 5% of the entire projected image as inthe case of the embodiment described above.

As described above, when the image signals input into the liquid crystalpanels 3 are at a low intensity level, the image quality can be improvedby reducing the size of the opening of the aperture 16 of the diaphragmmechanism 14 and increasing the intensity level of the image signals soas to increase the contrast while maintaining the illuminance of theprojected image at the level identical to that prior to the reduction ofthe size of the opening of the aperture 16.

Reduction of the size of the opening of the aperture 16 reduces thequantity of the light that reaches the screen S via the liquid crystalpanels 3, resulting in reduced illuminance of the projected image aswell as darker perfect black portion. Therefore, increasing theintensity level of image signals input into the liquid crystal panels 3while maintaining the reduced size of the aperture increases theilluminance of the projected image, in other words the entire signalsexcept for the perfect black portion. As a result, a projected imagewith a high contrast is obtained.

How the intensity of the image signals and the size of the opening ofthe aperture 16 should be controlled in order to improve the overallimage quality including contrast, in other words how much the size ofthe opening of the aperture 16 should be reduced and to what degree theintensity level of the image signals should be increased when theintensity of the image signals is a certain degree lower than themaximum level, depends on various factors, such as distribution of theintensity of the image signals, the brightness of a lamp serving as thelight source, etc. Therefore, the control criteria are not limited tothose described in the foregoing paragraphs; they should be determinedby a sensory test or other appropriate means.

Next, the fourth embodiment of the projector 1 is explained, referringto FIG. 4.

The fourth embodiment calls for detecting an object between theprojection lens 10 and the screen S and opening or closing the aperture16 of the diaphragm mechanism 14 in accordance with the presence orabsence of such an object.

To be more specific, an object detecting sensor 27 is connected to adiaphragm mechanism drive control circuit 22, which serves to adjust thesize of the aperture 16 of the diaphragm mechanism 14 by controlling thediaphragm mechanism 14, so that the diaphragm mechanism drive controlcircuit 22 receives signals from the object detecting sensor 27. Theobject detecting sensor 27 may be, for example, a so-called infraredtriangulation device, which comprises a light emitting section and alight receiving section for infrared light. Should signals from theobject detecting sensor 27 indicate presence of an object within adistance of, for example, 50 cm in front of the projection lens 10, thediaphragm mechanism drive control circuit 22 drives the motor to reduceor close the size of the opening of the aperture 16 of the diaphragmmechanism 14.

As described above, when there is an object between the projection lens10 and the screen S, the influence that light exerts on the object canbe prevented by closing off the aperture 16 of the diaphragm mechanism14. Should, for example, a person look in the projection lens 10 in thecourse of projecting an image, the diaphragm mechanism 14 closes off theaperture 16 to prevent the projected light from directly entering thehuman eye. The present embodiment is thus capable of limitingundesirable influence of light on a human eye or the like.

Next, the fifth embodiment of the projector 1 is explained, referring toFIG. 5.

The fifth embodiment calls for comparing the illuminance of the ambientlight around the projector 1, in other words, for example, theilluminance in the room, with the illuminance of a projected image onthe screen S, and, in accordance with the difference between theseilluminances, determining the size of the opening of the aperture 16 ofthe diaphragm mechanism 14 as well as the intensity level of the imagesignals input to the liquid crystal panels 3 so as to improve the imagequality.

The structure according to the fifth embodiment includes a firstphotoelectric element 31, a second photoelectric element 32, anilluminance comparison circuit 33, and a motor/image signal controlcircuit 35. The first photoelectric element 31 serves as an ambientlight illuminance detecting means for measuring ambient lightilluminance, i.e. the illuminance in the room in which projection isperformed. The second photoelectric element 32 serves as a projectedimage illuminance detecting means for measuring illuminance of an imageprojected on the screen S. The illuminance comparison circuit 33 servesas an illuminance comparing means for comparing an illuminance detectedby the first photoelectric element 31 and an illuminance detected by thesecond photoelectric element 32. The motor/image signal control circuit35 serves as a diaphragm mechanism/image signal control means forcontrolling intensity of image signals input into the liquid crystalpanels 3 and the size of the opening of the aperture 16 of the diaphragmmechanism 14 in accordance with a result of comparison by theilluminance comparison circuit 33. The motor/image signal controlcircuit 35 also functions as an image signal control circuit 21, whichserves as an image signal control means, and a diaphragm mechanism drivecontrol circuit 22, which serves as a diaphragm mechanism drive controlmeans. In order to control the diaphragm mechanism 14 and the liquidcrystal panels 3 in accordance with illuminance differences resultingfrom comparison, the motor/image signal control circuit 35 includes acorrelation table which stores predetermined control criteria to controlthe size of the opening of the aperture 16 of the diaphragm mechanism 14and the intensity level of image signals according to illuminancedifferences.

Based on signals from the two photoelectric elements 31,32, theilluminance comparison circuit 33 compares the illuminance in the roomand the illuminance of the image projected on the screen S. Based on theilluminance difference resulting from the comparison and referring tothe correlation table, the motor/image signal control circuit 35determines the size of the opening of the aperture 16 of the diaphragmmechanism 14 and the intensity level of the image signals. In accordancewith the determined values, the motor/image signal control circuit 35drives the motor of the diaphragm mechanism 14 and changes the intensitylevel of the image signals. The contents of the aforementionedcorrelation table are set so that, when the illuminance of the projectedimage on the screen S is not intense enough with respect to theilluminance of the light in the room, the motor/image signal controlcircuit 35 increases the intensity of the image signals and drives themotor in such a direction as to reduce the size of the opening of thediaphragm mechanism 14.

When the illuminance of an image projected on the screen S is notsufficiently intense with respect to the illuminance in the room, thedifference in level of illuminance between the projected image and theperfect black portion is small, resulting in the projected image withlow contrast. With the configuration of the embodiment described above,however, the image quality can be improved in such a situation byreducing the size of the opening of the diaphragm mechanism 14 toincrease the contrast of the project image and setting the intensity ofimage signals at a high level to prevent reduction in illuminance of theprojected image regardless of the reduced size of the opening of thediaphragm mechanism 14.

The contrast of a projected image is dependent on not only such factorsas stray light inside the lens barrel 11 and the aberration of the lensbut also other factors, including a sensory aspect, such as anobserver's response to the brightness of the illuminance in the room andthe illuminance of a projected image on the screen S. Therefore,specific criteria in the correlation table, which determines therelationship between the size of the opening of the diaphragm mechanism14 and the intensity level of an image signal, should be set, takinginto consideration results of sensory tests or the like.

The embodiment described above calls for detecting both the level ofilluminance in the room and the illuminance of a projected image on thescreen S. However, as the illuminance in the room exerts an greaterinfluence on the contrast of a projected image than does the illuminanceof the projected image on the screen S, the contrast of the projectedimage can be improved while maintaining the same level of illuminance bydetecting the level of illuminance in the room alone and setting thesize of the opening of the diaphragm mechanism 14 and the intensitylevel of image signals in accordance with the detected illuminance inthe room.

The higher the level of illuminance in the room, the smaller thedifference in level of illuminance between the light in the room and theprojected image, resulting in reduced contrast of the projected image.In an alternative arrangement, contrast of a projected image may beimproved by increasing the size of the opening of the diaphragmmechanism 14 as well as the illuminance of the projected image when thelevel of illuminance in the room is higher than a preset level or whenthe difference in level of illuminance between the light in the room andthe projected image is smaller than a preset value. Although increasingthe size of the opening of the diaphragm mechanism 14 makes the contrastof the project image prone to reduction, the reduction in the contrastcan be limited by determining the shape of the aperture 16 of thediaphragm mechanism 14 in accordance with the field anglecharacteristics of the liquid crystal panels 3.

In either one of the second embodiment or the fifth embodiment, theprojection lens 10 may be arranged telecentric to rays of light emittedfrom the liquid crystal panels 3 with the aperture 16 of the diaphragmmechanism 14 positioned so that the principal rays A intersect with theoptical axis B. Arranging the projection lens 10 as described abovepermits rays of light emitted from various locations on the liquidcrystal panels 3 to be shut out in a symmetrical manner with respect tothe optical axis B, thereby limiting reduction in telecentric propertiesand, consequently, easily ensuring both telecentricity and improvedcontrast. Furthermore, by forming the aperture 16 of the diaphragmmechanism 14 in such a shape as to ensure a sufficiently high contrastin accordance with contrast characteristics with respect to directionsand angles of incidence to the liquid crystal panels 3, contrast can beeasily improved, compared with a structure that includes a circularaperture, while reduction in illuminance of a projected image isprevented.

Next, an explanation is given on the field angle characteristics of theliquid crystal panels 3. FIGS. 6 through 9 show measuring methods, andFIG. 10 shows results of measurements.

The measurement was performed under the following basic conditions:driving voltages of HV_(DD)=15.5 V, VV_(DD)=15.5 V, VVC_(DD)=7.0 V, andV_(com)=6.6 V, a temperature of 25 □ the measurement point being onepoint at the center of the picture frame, a measurement system I and ameasurement system II described below as the measurement systems, andvideo input signal voltages with respect to a signal amplitude beingV_(sig)=7.0 V±VAC[V].

FIG. 6 shows the measurement system I, wherein an image projected on thescreen S, which is disposed at a location of A=2000 mm from theprojector 1, is measured by means of a measuring device 42 via aluminance meter 41. The screen S is of a glass beads type with a gain of2.8. The projection lens 10 of the projector 1 has a focal length of 80mm and a F-number of F1.9. The light source is a 155W metal halide lampwith a color temperature of 7500 K±500. The magnification is 24×, andthe measurement diameter is 7 mm φ. EG-EG1224DU (product name of Nitto),SKN-18242T (product name of Polatechno), or a product having equivalentspecifications is used as the polarizing plate.

FIGS. 7 and 8 show the measurement system II, wherein the liquid crystalpanels 3 are driven by a driving circuit 44, and light emitted from amovable light source W is passed through the liquid crystal panels 3 andreceived by a light receiving lens 46, which is disposed so as to facetowards the light source and be capable of moving around the center ofthe liquid crystal panels 3. The light receiving lens 46 is connected toa light detecting device 48 via an optical fiber 47. The light detectingdevice 48 is connected to a measuring device 49.

In FIG. 8, reference code 3 a represents the main body portion of theliquid crystal panels 3, 3 b a frame portion surrounding the main bodyportion, 3 c a flexible wiring, and 3 d a mark.

FIG. 9 is a graph showing the relationship between optical angles (°) oflight that strikes the liquid crystal panels 3 and relative lightintensities.

In FIG. 10, reference code CR represents a contrast shown in the term ofratio. A contrast CR is found by measuring two factors in themeasurement system I, i.e. a surface brightness L (white) at the centerof the screen S when the input signal amplitude (V_(AC)) is 0.5 V and asurface brightness L (black) at the center of the screen S when theinput signal amplitude (V_(AC)) is 4.5 V, and computing CR based on theequation CR=L(white)/L(black).

The liquid crystal panels 3 functions as a light shutter by controllingthe arrangement of liquid crystal molecules and permitting changes inthe arrangement to be seen by means of combination with a polarizingplate. The characteristics of the liquid crystal panels 3 vary dependingon angles of incidence of light. In particular, with regard to contrastof light that has passed through the liquid crystal panels 3, thecharacteristics of the liquid crystal panels 3 are dependent on not onlythe angle of incidence but also the direction of incidence. To be morespecific, generally speaking, the higher the angle of light entering theliquid crystal panels, in other words the closer to a right angle (the θwith respect to the Z axis is small), the higher the contrast. For thisreason, it is a common practice to improve contrast of an image byreducing the diameter of a circular aperture so as to shut out lightthat enters at a low incident angle and therefore would reduce thecontrast. However, the characteristics of the liquid crystal panels 3vary also with direction of incidence, i.e. (p with respect to the Yaxis. As is evident from the result of the measurement shown in FIG. 10,in a part having a relatively high contrast, lines that connect areashaving the same level of contrast form a substantially triangular shape,while lines that connect areas having the same level of contrast in apart having a relatively low contrast form a square with four curvedsides.

In order to solve the above problem, the shape of the part having arelatively high contrast is taken into consideration in the firstembodiment shown in FIG. 2 so that the aperture 16 is formed in asubstantially triangular shape. In other words, the aperture 16 of thediaphragm mechanism 14 is formed in a shape appropriate for the fieldangle characteristics so that the opening of the aperture 16 correspondsto the part through which light beams having a high contrast pass. Bythus forming the aperture 16 in such a shape as to substantiallyincrease the proportion of the light passing through the aperture 16that has a high contrast, it is possible to ensure that a projectedimage has both a sufficiently high contrast and intense illuminance.Unlike a circular aperture, which is inevitably subject to the problemof a reduced illuminance when reducing the diameter of the aperture topermit only the light that has a high contrast to pass through, thestructure of the first embodiment permits a large amount of the lighthaving a high contrast to pass through the diaphragm mechanism 14 whilemaintaining a large aperture diameter, which corresponds to the size ofthe opening of the diaphragm mechanism 14 through which light passes.The embodiment is thus capable of simultaneously achieving both a highcontrast and a sufficiently intense illuminance and thereby easilyimproving the performance characteristics of the projector.

In other words, as the aperture 16 is comprised of a center portion,which has a conventional circular shape, and projecting portions C, eachof which extends from the center portion along a part having a highcontrast, the illuminance of an image is intensified while reduction incontrast is limited. The embodiment is thus capable of simultaneouslyachieving both a high contrast and a sufficiently intense illuminanceand thereby easily improving the performance characteristics of theprojector.

In order to improve the illuminance of a projected image as much aspossible and obtain a simple shape so as to reduce the production costs,the first embodiment shown in FIG. 2 takes into consideration the shapeof the part having a relatively high contrast and calls for forming theaperture 16 in a substantially triangular shape that includes projectingportions C projecting in three directions. However, the invention is notlimited to such a configuration; the aperture 16 may be formed in anyother appropriate shape.

An example of other configurations of the aperture is offered by a sixthembodiment shown in FIG. 11, which takes into consideration the shape ofthe part having a relatively high contrast and aims at improvingcontrast rather than illuminance. In order to achieve this objective aswell as obtain a simple shape at reduced production costs, the sixthembodiment offers a substantially triangular aperture 51 havingprojecting portions C that project in three directions.

Another example is offered by a seventh embodiment shown in FIG. 12,which also takes into consideration the shape of the part having arelatively low contrast and aims at improving illuminance as much aspossible. In order to achieve this objective, the seventh embodimentoffers a substantially trapezoidal aperture 53 having projectingportions C that project in four directions.

Yet another example is offered by an eighth embodiment shown in FIG. 13,which also takes into consideration the shape of the part having arelatively low contrast and aims at improving contrast and illuminanceas much as possible. In order to achieve this objective, the seventhembodiment offers a substantially trapezoidal aperture 55 of which thefour sides curve inward approximately along lines that connects areashaving the same level of contrast, with projecting portions C projectingin four directions.

Yet another example is offered by a ninth embodiment shown in FIG. 14,which also takes into consideration the shape of the part having arelatively low contrast and aims at improving contrast and illuminanceas much as possible and averaging out the quantity of light of aprojected image. In order to achieve this objective, the seventhembodiment offers a substantially square aperture 57 of which the foursides curve inward approximately along lines that connects areas havingthe same level of contrast, with projecting portions C projecting infour directions.

Should the field angle characteristics of the liquid crystal panels 3serving as the display device differ, the aperture 16 of the diaphragmmechanism 14 may have a different shape in accordance with thecharacteristics of the liquid crystal panels 3.

Although the invention is explained as above referring to theembodiments wherein the transmission type liquid crystal panels 3 areused as the display device, the invention is not limited to such afeature; the invention is applicable to various display devices that usea plurality of pixels.

As a result of provision of a diaphragm mechanism, the lens device ofthis invention is capable of increasing the F-number (in other wordsreducing the view angle) so as to shut out the light that passes nearthe perimeter of the lens, which is the area prone to the influence oflens aberration. By thus forming a projection image of the light thatpasses near the center of the lens and is therefore less prone to lensaberration, the lens device claimed in claim 1 is capable of improvingthe contrast of the projected image. As the lens device is arrangedtelecentric to rays of light emitted from the display device, theprincipal rays of light emitted from various locations of the displaydevice converge at a single point and intersect with the optical axis.Providing the diaphragm mechanism with an aperture at such a positionthat the principal rays intersect with the optical axis permits rays oflight emitted from various locations on the display device to be shutout in a symmetrical manner with respect to the optical axis, therebylimiting reduction in telecentricity and, consequently, easily ensuringboth telecentricity and improved contrast.

The shape of the aperture of the diaphragm mechanism of the lens deviceis determined in accordance with contrast characteristics with respectto directions and angles of incidence to the display device so as toensure a sufficiently high contrast. Therefore, this lens device iscapable of limiting reduction in illuminance of a projected image andalso easier to improve the contrast of a projected image than is astructure that includes a circular aperture. The diaphragm mechanism ofthe lens device may be designed such that the size of the opening of itsaperture is adjustable, thus facilitating the adjustment of the contrastand the brightness of a projected image. As a result of inclusion ofsuch a lens device, a projector claimed in claim 4 easily ensures bothtelecentricity and improved contrast.

Should the intensity of image signals input into the display device beat a low level, a projector according to this invention is capable ofincreasing the contrast of a projected image while maintaining itsilluminance by reducing the size of the opening of the aperture of thediaphragm mechanism and increasing the intensity of the image signals.

Should there be an object between the lens device and the screen, such aprojector is capable of limiting the impact light has on the object byreducing the dimension of the aperture of the diaphragm mechanism.

Should the illuminance of an image projected on the screen is notsufficiently intense with respect to the level of illuminance in theroom, such a projector is capable of reducing the size of the opening ofthe diaphragm mechanism so as to increase the contrast of the projectedimage and setting the intensity of the image signals at a high level soas to limit reduction in the illuminance of the projected image.

As the lens device of a projector of this invention may be arranged soas to be telecentric to rays of light emitted from the display device,the principal rays of light emitted from various locations of thedisplay device converge at a single point and intersect with the opticalaxis. Therefore, by providing the diaphragm mechanism with an apertureat such a position that the principal rays intersect with the opticalaxis, this projector also permits rays of light emitted from variouslocations on the display device to be shut out in a symmetrical mannerwith respect to the optical axis, thereby limiting reduction intelecentricity and, consequently, easily ensuring both telecentricityand improved contrast.

The shape of the aperture of the diaphragm mechanism of such a projectoris determined in accordance with contrast characteristics with respectto directions and angles of incidence to the display device so as toensure a sufficiently high contrast. Therefore, such a projector is alsocapable of limiting reduction in illuminance of a projected image andalso easier to improve the contrast of a projected image than is astructure that includes a circular aperture and may facilitate theimprovement of image quality by improving the contrast of the imagewhile ensuring telecentricity even in cases where the projector includesliquid crystal panels whose contrast characteristics are polarized withrespect to the direction and angle of incidence.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

1. A projector comprising: a light source; a display device to modulatelight emitted from the light source; a lens device to project lightemitted from said light source via said display device, wherein saidlens device comprises a diaphragm mechanism formed having an aperture;an ambient light illuminance detector to measure illuminance of ambientlight; a projected image illuminance detector to measure illuminance ofan image projected on the screen; an illuminance comparator to comparean illuminance detected by said ambient light illuminance detector andan illuminance detected by said projected image illuminance detector;and a diaphragm mechanism/image signal controler to control an intensityof image signals input into the display device and to control a size ofan opening of the aperture of the diaphragm mechanism in accordance witha result of comparison by the illuminance comparator.
 2. The projectoras claimed in claim 1, wherein the lens device is arranged telecentricto rays of light emitted from the display device and comprises adiaphragm mechanism having an aperture at such a position that the lensdevice causes principal rays of light emitted from various location onthe display device to intersect with the optical axis.
 3. The projectoras claimed in claim 2, wherein the shape of the aperture of thediaphragm mechanism is determined in accordance with contrastcharacteristics with respect to directions and angles of incidence oflight emitted from the display device onto the lens device so as toensure a sufficiently high contrast.
 4. The projector as claimed inclaim 1, wherein the display device is comprised of liquid crystalpanels.